As a multiplexing system in optical fiber communication, there is wavelength division multiplexing system. If a signal strength of each wavelength channel of a wavelength division multiplexing signal is uneven, S/N of each wavelength channel of the wavelength division multiplexing signal is uneven, and a sufficient system margin cannot be secured. Thus, the signal strength of each wavelength channel of the wavelength division multiplexing signal is measured using an array-type photo module. Then, the signal strength of each wavelength channel of the wavelength division multiplexing signal is adjusted based on the measuring result, using an erbium-doped optical fiber amplifier.
A photo module measuring the signal strength of a single wavelength channel is disclosed in Patent Documents 1 to 4. The array-type photo module measuring the signal strength of a plurality of the wavelength channels can be manufactured by arraying these photo modules.
A line monitor disclosed in Patent Document 1 is constituted of an incident optical fiber, an emitting optical fiber, a rod lens, a reflective film including a light-transmitting portion, and a received light detection element. An optical signal of a single wavelength channel reaches the reflective film through the incident optical fiber and the rod lens. The optical signal reaching the light-transmitting portion of the reflective film is detected by the received light detection element. The optical signal reaching a reflective portion of the reflective film is output to a transmission path through the rod lens and the emitting optical fiber.
Optical transmitter and receiver modules disclosed in Patent Document 2 is constituted of a light-emitting element, a light-receiving element, a wavelength selection filter, an optical fiber, and a wall surface including a drilled hole. A transmission optical signal is generated in the light-emitting element to be rendered monochromatic in the wavelength selection filter, and, thus, to be sent to the optical fiber. A reception optical signal is received from the optical fiber to be reflected by the wavelength selection filter, and, thus, to be detected by the light-receiving element. The transmission optical signal could be input to the light-receiving element when reflected by the wavelength selection filter. Thus, the drilled hole is arranged on the wall surface between the light-emitting element and the wavelength selection filter, whereby the transmission optical signal reflected by the wavelength selection filter is reflected by the drilled hole of the wall surface to thereby be prevented from being input to the light-receiving element.
An unidirectional optical power monitor disclosed in Patent Document 3 is constituted of an incident optical fiber, an emitting optical fiber, a GRIN lens, a tap film, an optical diode, and a sleeve. In the sleeve, the central axis of a circular hole into which the GRIN lens and the optical diode are inserted and mounted is deviated. An optical signal of a single wavelength channel reaches the tap film through the incident optical fiber and the GRIN lens. The optical signal transmitted through the tap film is detected by the optical diode. The optical signal reflected by the tap film is output to a transmission path through the GRIN lens and the emitting optical fiber. Although the optical signal from the incident optical fiber should be detected by the optical diode, return light from the emitting optical fiber may be detected by the optical diode. Thus, an intermediate wall is arranged at a connecting position of the circular hole into which the GRIN lens and the optical diode are inserted and mounted, whereby the return light from the emitting optical fiber is reflected by the intermediate wall to thereby be prevented from being input to the optical diode.
An optical monitor module disclosed in Patent Document 4 is constituted of an incident optical fiber, an emitting optical fiber, a lens portion for incidence, a lens portion for emission, a beam splitter, and a photo diode. An optical signal of a single wavelength channel reaches the beam splitter through the incident optical fiber and the lens portion for incidence. The optical signal transmitted by the beam splitter is detected by the photo diode. The optical signal reflected by the beam splitter is output to a transmission path through the lens portion for emission and the emitting optical fiber. Since the optical signal emitted from the lens portion for incidence is guided in the direction of the beam splitter, the inclination angle with respect to the central axis of the lens portion for incidence at the emitting end of the lens portion for incidence is adjusted. Since the optical signal reflected by the beam splitter is guided in the direction of the emitting optical fiber, the inclination angle with respect to the central axis of the lens portion for emission at the incident end of the lens portion for emission is adjusted.